Draw lines along the grid edges to divide the grid into connected regions, so that every region contains exactly one of each letter. Every letter appears the same number of times in the grid, so the regions all come out the same size: a 6×6 grid using four letters divides into nine four-square regions, each holding one A, one B, one C and one D.
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click).
Click an edge to mark it as a division between regions (black), and again to return it to undecided (yellow). Right-click it to mark it as definitely not a division (faint gray), and again to return it to undecided. Keep the button down and drag to do the same to every edge you pass over that looked the same as the first.
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the edge under it, and Space what a right-click does.
The cursor moves by half a square, so it rests on the edges between squares as well as on the squares themselves.
A region turns purple once it is complete and correct. Lines that close off a region of the wrong size are shown in red, as is a letter that appears twice within one region.
Use the Type menu to change the size of the grid and how many different letters it uses.
Hint explains the next step rather than simply making it. A region, in a hint’s sentence, is a group of squares your own “no wall” marks already join. A square they join to nothing is reasoned about in the same way.
When a hint talks about two regions, one is striped and the other outlined, so you can tell them apart even where they touch: “the striped and outlined regions both hold an A” means those two regions, and no others. A single square is named by its letter (“these two Ds”) or called a square (“the striped square and the outlined region”), never a region.
On an Unreasonable board there comes a point where no two neighbors share a letter that is not already walled, and no region has only one square left to take. The hint then says that nothing further follows by deduction instead of choosing for you. Save your position and try joining a small region to one of the squares it could still take, starting with a region that has few of them: if it is wrong, before long some region is left short with every square around it walled off. The hint carries on from whatever you decide.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.